Christopher Galley, the 26-year-old civil servant at the centre of the Damian Green affair, regularly leaked documents to the Tory frontbencher for nearly two years because he believed they would be "used in a responsible manner in the public interest", his lawyer said last night.

Galley's solicitor, Neil O'May, insisted the information involved was of the "lowest level" and attacked the nature of the police investigation, which included the Home Office official being held for 17 hours after a dawn raid on his home by anti-terrorist officers two weeks ago.

Galley's appearance came as Ken Jones, the president of the Association of Chief Police Officers, defended the Metropolitan police action in arresting Green, a shadow Home Office spokesman, saying that no person, regardless of their station, should be given the opportunity to influence an inquiry in their favour.

David Cameron is expected to ask two Conservative heavyweights, Ken Clarke and Michael Howard, to lead his party's assault on Gordon Brown and the Commons Speaker over Green's arrest when MPs return to Westminster tomorrow.

O'May, who acted for Lord Levy during the "cash for honours" affair, sat alongside Galley at yesterday's press conference and said his client was unable to speak because he was still on bail and under police investigation: "If ever there is a case of 'don't shoot the messenger', this is it," said O'May. He added that Galley had been open and cooperative and had volunteered the whereabouts of his computer, mobile phone and the documents the police needed for their inquiries.

O'May said Galley first met Green in the Commons in 2006. "He was in contact with Mr Green over the following two years, including further meetings with him. As a result, Mr Galley gave Damian Green MP information which was important for the public to know in an open and democratic parliamentary system."

He confirmed the systematic nature of the leaks, which it has been claimed amounted to more than 20 documents, by admitting they had been "close to regular" over the past two years.

O'May tried to play down their significance, saying Galley had first met Green because of his concerns over immigration and the documents amounted to "embarrassment material". He stressed that Galley had not been charged under the Official Secrets Act. "It's not really state secrets, national security, terrorism, financial jeopardy, loss, gain or otherwise - nothing remotely like that," he said.

O'May would not comment on whether or not inducements had been offered to Galley, or on reports that his client had stood as a Tory candidate and then been interviewed by Green for a Commons job before joining the Home Office.

He did however dismiss speculation that Galley had been involved in a police plot after his arrest to set up Green, describing the allegation as a "malicious rumour and wholly untrue".

Cameron and his circle believe the arrest of Green marks a defining moment of what they describe as "state heavy-handedness". But they want to act cautiously in order to be seen as a government in waiting and because they do not want to lose sight of their main prey: Gordon Brown.

Conservative plans are to be finalised at a special meeting of the backbench 1922 committee. Clarke is expected to play a leading role in challenging the Speaker, Michael Martin, and the prime minister. Howard is also expected to be on hand, though he has less cross-party appeal.

Their attack is expected to:

• Challenge the Speaker, who will make a statement, to explain how the Commons authorities allowed police to enter the Palace of Westminster and search Green's office. The Tories are suggesting that the Commons authorities may have been misled by the police, who reportedly said their action had been approved by the Director of Public Prosecutions, Keir Starmer. Starmer said he was informed just before the arrest took place.

• Maintain the pressure on Gordon Brown by asking the famous Watergate question: who knew what when? The Tories believe that home secretary Jacqui Smith made a grave mistake in telling her officials she did not want to know about any police action against Green.

Amid cabinet concerns at the conduct of the police, Downing Street indicated that there may be grounds for an inquiry once the investigation is over. "There may well be a time when it is right to look at any specific issues arising from this case," said a spokesman for Brown. "But we can't do that without knowing what the facts of this case or in a way that might undermine the operational independence of the police."

Jack Straw, the justice secretary, indicated that he was surprised by the arrest of Green and search of his homes and properties. He said: "Everybody who has been made aware of these unfolding events of course is surprised by the nature of the raid, and I certainly understand the concern of my parliamentary colleagues."